On default Gnome Desktop, users can restart X using Ctrl+Alt+Backspace. To avoid it you can try this tips.
First you need to backup your xorg.conf file using the following command :

$ sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup

Now you need to edit the file using the following command :

$ sudo vi /etc/X11/xorg.conf

Add the following lines at the end of xorg.conf file :

Section “ServerFlags”

Option “DontZap” “yes”

EndSection

Save the edited file using :x

Now users can't restarting your X using Ctrl+Alt+Backspace :)